Boris Johnson may have a long list of people to thank today, but one name is unlikely to crop up in public alongside those of David Cameron, George Osborne and Lynton Crosby.

That name is Veronica Wadley, the editor of the London Evening Standard and, some would argue, the single most powerful figure in his campaign team.

Wadley backed Johnson before he was chosen as the Tory candidate and gave him a proverbial kick up the backside for running a lacklustre campaign. For the past six months, her paper has mounted attacks on Ken Livingstone, his friends, his advisers and his years in office, opening the paper to criticism that it has been pushing a very personal political agenda.

Even Standard insiders admit to being surprised at the vigour with which the plan to get Johnson elected has been pursued. "This is London's only paid-for newspaper. If you were talking about a city with three newspapers it might feel a bit less odd," said Prof Adrian Monck, head of journalism and publishing at City University. "In a city with just one newspaper, the fact it backs one candidate so wholeheartedly feels odd, it feels partisan. It's quite a bold move, but it will be able to say it was the Standard wot won it."

The newspaper's key contribution may have been the fact it could put around 500 billboards on London streets every day declaring that Livingstone was corrupt and Johnson was heading for victory.

On Wednesday the paper concluded its coverage with a full-page leading article backing Johnson under a picture of him pointing Kitchener-like at the camera. The headline was: "Honesty and competence". The Standard's Andrew Gilligan, who was employed by Johnson at the Spectator magazine after resigning from the BBC in the wake of the Hutton report, argues that Livingstone is an entirely legitimate target. Earlier this week he told the Guardian: "I'm not working to get a Tory elected. I'm working to get Ken unelected."

It was Gilligan's story about alleged malpractice by Livingstone's race adviser, Lee Jasper, that set the tone of the Standard's coverage.

"If ever a single story has done for a politician it may just be that one," said Charlie Beckett, director of the media thinktank Polis at the London School of Economics. "It's like the Kinnock rally thing [in

Sheffield before the 1992 general election]: it confirms what you suspected might be true in the first place." If nothing else, it has set the terms of debate and provided a background hum that Livingstone's administration has grown tired.

The Standard may have another motivation for backing Johnson. In 2010, two years into his administration, Associated's contract to supply London underground stations with its free Metro newspaper will run out. The contract's renewal is a decision for Transport for London, whose chairman the mayor appoints. Metro has proved a success for Associated and it is determined to defend it.

Wadley's supporters argue that Livingstone has antagonised Associated by likening its Jewish reporter Oliver Finegold to a Nazi concentration camp guard and telling him that the paper was "a load of scumbags and reactionary bigots" with "a record of supporting fascism".